Ink jet printing is a non-impact method of printing that involves ejecting ink from a nozzle onto paper or other print media. The actual ink ejection method may occur via several processes including pressurized nozzles, electrostatic fields, piezoelectric elements within an ink nozzle, and heaters for vapor phase bubble formation.
The composition of the ink is traditionally comprised of deionized water, a water soluble organic solvent, and a colorant. The colorant may be a soluble dye or insoluble pigment. Several problems, however, are associated with soluble dyes that are not applicable to insoluble pigments. These problems include poor water-fastness, poor light-fastness, poor thermal stability, facile oxidation, dye crystallization, and ink bleeding and feathering on the print medium. To circumvent these problems, use of a pigment as the colorant is preferred. Pigments generally have better light-fast and water-fast properties, are more resistant to oxidation, and have higher thermal stability. However, pigment based inks have some disadvantages as compared to dye-based inks. Due to the insolubility of pigments in water, polymeric dispersants and/or surfactants are typically added to improve the dispersibility of the pigment. Also, because pigments exist as discrete particles in ink, pigments have a strong light scattering effect. As a result, pigmented inks have a lower chroma as compared to dye based inks and often look dull when printed on plain paper. Fiber show through is also a problem when using pigment based inks.
Several methods have been used in an attempt to improve the chroma, dispersability, and performance of pigment based inks. For example, in attempts to improve the chroma of a pigment based ink, additives such as optical brighteners and/or fluorescent whiteners have been added to the ink formulation. Dyes can also be added to pigment based inks in the form of a dyed pigment, polymeric dyes, a polymer resin with an oil soluble dye, or a complex product of dye and polymer. Methods to improve the chroma, dispersability, and performance of pigment based inks may solve some of these problems, but at the same time, other physical qualitities of the ink can be sacrificed.
Further, in an effort to improve color and other print qualities, the drop mass and the nozzle diameter of ink jet print heads has been reduced. To accommodate smaller drop mass and nozzle diameters and simultaneously maintain chroma and performance, pigment based inks require pigment dispersions with improved stability.
Thus, there is a need for pigment based inks comprising a pigment dispersion with improved stability and good chroma.